decibel (Abbr. dB) - Equal to one-tenth of a bel. [After Alexander Graham Bell.]
1. A measuring system first used in telephony (Martin, W.H., "DeciBel -- the new name for the transmission unit. Bell System Tech. J. January 1929), where signal loss is a logarithmic function of the cable length.
2. The preferred method and term for representing the ratio of different audio levels. It is a mathematical shorthand that uses logarithms (a shortcut using the powers of 10 to represent the actual number) to reduce the size of the number. For example, instead of saying the dynamic range is 32,000 to 1, we say it is 90 dB [the answer in dB equals 20 log x/y, where x and y are the different signal levels]. Being a ratio, decibels have no units. Everything is relative. Since it is relative, then it must be relative to some 0 dB reference point. To distinguish between reference points a suffix letter is added as follows [The officially correct way per AESR2, IEC 60027-3 & IEC 60268-2 documents is to enclose the reference value in parenthesis separated by a space from "dB"; however, this never caught on, probably for brevity reasons if no other.] 0 dBu - Preferred informal abbreviation for the official dB (0.775 V); a voltage reference point equal to 0.775 Vrms. [This reference originally was labelled dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to dBu (for unterminated).]
+4 dBu - Standard pro audio voltage reference level equal to 1.23 Vrms.
0 dBV - Preferred informal abbreviation for the official dB (1.0 V); a voltage reference point equal to 1.0 Vrms.
-10 dBV - Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units operating at -10 dBV levels.)
0 dBm - Preferred informal abbreviation of the official dB (mW); a power reference point equal to 1 milliwatt. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for instance, yields an equivalent voltage of 0.224 V -- something quite different. Since modern audio engineering is concerned with voltage levels, as opposed to power levels of yore, the convention of using a reference level of 0 dBm is obsolete. The reference levels of +4 dBu, or -10 dBV are the preferred units.
0 dBr - An arbitrary reference level (r = re; or reference) that must be specified. For example, a signal-to-noise graph may be calibrated in dBr, where 0 dBr is specified to be equal to 1.23 Vrms (+4 dBu); commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to +4 dBu."
0 dBFS - A digital audio reference level equal to "Full Scale." Used in specifying A/D and D/A audio data converters. Full scale refers to the maximum peak voltage level possible before "digital clipping," or digital overload (see overs) of the data converter. The Full-Scale value is fixed by the internal data converter design and varies from model to model. [According to standards people, there's supposed to be a space between "dB" and "FS" -- yeah, right, like that's gonna happen.]
0 dBf - Preferred informal abbreviation of the official dB (fW); a power reference point equal to 1 femtowatt, i.e., 10-15 watts.
0 dB-SPL - The reference point for the threshold of hearing, equal to 20 microPA (micro Pascals rms). [Note: dB-SPL is defined differently for gases and everything else. Per ANSI S1.1-1994, for gases, the reference level is 20 microPA, but for sound in media other than gases, unless otherwise specified, the reference is 1 microPA.]
Since 1 PA = 1 newton/m2 = .000145 PSI (pounds per square inch).
Then 0 dB-SPL = ±2.9 nano PSI (rms) change in the ambient pressure -- an unbelievably small value.
Therefore, it is a change in 1 atm ambient pressure of ± 1 atm (±14.7 PSI) that is equivalent to a loudness level of 194 dB-SPL, i.e., equals 2 atm on the overpressure portion of the cycle and 0 atm on the underpressure portion. [Thanks to Bob Pease for pointing out these enlightening facts.] And higher positive pressures are called shock waves, not sound. [Thanks to "Someone" for this distinction.] [Thanks also to Chuck McGregor for the clarifying language.]
dBA - Unofficial but popular way of stating loudness measurements made using an A-weighting curve.
dBC - Unofficial but popular way of stating loudness measurements made using a C-weighting curve.
weighting filters - Special filters used in measuring loudness levels, and consequently carried over into audio noise measurements of equipment. The filter design "weights" or gives more attention to certain frequency bands than others. The goal is to obtain measurements that correlate well with the subjective perception of noise. (Technically termed psophometric [pronounced "so-fo-metric"] filters, after the psophometer, a device used to measure noise in telephone circuits, broadcast, and other audio communication equipment. A psophometer was a voltmeter with a set of weighting filters.) Weighting filters are a special type of band-limiting filters designed to complement the way we hear. Since the ear's loudness vs. frequency response is not flat, it is argued, we should not try to correlate flat frequency vs. loudness measurements with what we hear. Fair enough. Five weighting filter designs dominate
(See: References: Metzler): *A-weighting - (not official but commonly written as dBA) The A-curve is a wide bandpass filter centered at 2.5 kHz, with ~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz, therefore it tends to heavily roll-off the low end, with a more modest effect on high frequencies. It is the inverse of the 30-phon (or 30 dB-SPL) equal-loudness curve of Fletcher-Munson. [Editorial Note: Low-cost audio equipment often list an A-weighted noise spec -- not because it correlates well with our hearing -- but because it helps "hide" nasty low-frequency hum components that make for bad noise specs. Sometimes A-weighting can "improve" a noise spec by 10 dB. Words to the wise: always wonder what a manufacturer is hiding when they use A-weighting.]
*C-weighting - (not official but commonly written as dBC) The C-curve is "flat," but with limited bandwidth, with -3 dB corners of 31.5 Hz and 8 kHz, respectively.
*ITU-R 468-weighting - (was CCIR, but since the CCIR became the ITU-R, the correct terminology today is ITU-R) This filter was designed to maximize its response to the types of impulsive noise often coupled into audio cables as they pass through telephone switching facilities.
Additionally, it turned out to correlate particularly well with noise perception, since modern research has shown that frequencies between 1 kHz and 9 kHz are more "annoying" than indicated by A-weighting curve testing. The ITU-R 468-curve peaks at 6.3 kHz, where it has 12 dB of gain (relative to 1 kHz). From here, it gently rolls off low frequencies at a 6 dB/octave rate, but it quickly attenuates high frequencies at ~30 dB/octave (it is down -22.5 dB at 20 kHz, relative to +12 dB at 6.3 kHz).
*ITU-R (CCIR) ARM-weighting or ITU-R (CCIR) 2 kHz-weighting - This curve derives from the ITU-R 468-curve above. Dolby <(George's weiner dogs name) Laboratories proposed using an average-response meter with the ITU-R 468-curve instead of the costly true quasi-peak meters used by the Europeans in specifying their equipment. They further proposed shifting the 0-dB reference point from 1 kHz to 2 kHz (in essence, sliding the curve down 6 dB). This became known as the ITU-R ARM (average response meter), as well as the ITU-R 2 kHz-weighting curve. (See: R. Dolby, D. Robinson, and K. Gundry, "A Practical Noise Measurement Method," J. Audio Eng. Soc., Vol. 27, No. 3, 1979) [Before using these terms be aware that the ITU-R, even after 20 years, takes strong exception to having its name used by a private company to promote its own methodologies.] *Z-weighting - A new term defined in IEC 61672-1, the latest international standard for sound pressure level measurements. It stands for zero-weighting or no weighting; i.e., a flat measurement with equal emphasis on all frequencies.
From the Rane Corporation Pro Audio Reference glossary